1. Field
This document relates to a liquid crystal display driven using a double rate driving (DRD) method, and more particularly, to a liquid crystal display which is capable of improving picture quality by compensating for difference in charge between liquid crystal cells.
2. Related Art
A liquid crystal display is configured to display images by controlling the light transmissivity of a liquid crystal layer using an electric field supplied to the liquid crystal layer in response to a video signal. The liquid crystal display is a flat panel display having the advantages of small size, slimness, and low power consumption, and is used in portable computers such as notebook PCs, office automation devices, audio/video devices, and so on. In particular, a liquid crystal display of an active matrix type in which a switching element is formed in each liquid crystal cell is advantageous for implementing motion pictures because it can actively control the switching elements.
A thin film transistor (hereinafter referred to as a “TFT”), as shown in FIG. 1, is generally used as the switching element for the active matrix type liquid crystal display.
Referring to FIG. 1, the active matrix type liquid crystal display is configured to convert digital video data into an analog data voltage on the basis of a gamma reference voltage, supply the converted data voltage to a data line DL and, at the same time, supply a scan pulse to a gate line GL, thereby charging a liquid crystal cell Clc with the data voltage. To this end, the gate electrode of a TFT is coupled to the gate line GL, the source electrode of the TFT is coupled to the data line DL, and the drain electrode of the TFT is coupled to the pixel electrode of the liquid crystal cell Clc and one of the electrodes of a storage capacitor Cst. A common voltage Vcom is supplied to the common electrode of the liquid crystal cell Clc. When the TFT is turned on, the storage capacitor Ct is charged with the data voltage supplied from the data line DL, thus functioning to constantly maintain the voltage of the liquid crystal cell Clc. When the scan pulse is supplied to the gate line GL, the TFT is turned on and a channel is formed between the source electrode and the drain electrode, so the voltage of the data line DL is supplied to the pixel electrode of the liquid crystal cell Clc. At this time, the arrangement of liquid crystal molecules of the liquid crystal cell Clc is changed by an electric field between the pixel electrode and the common electrode, so light incident on the liquid crystal cell is changed.
This liquid crystal display comprises a gate drive integrated circuit (IC) for driving the gate lines GL, and a data drive IC for driving the data lines DL. As the size and definition of liquid crystal displays increase, so does the required number of drive ICs. Because the data drive ICs are more expensive than other elements, several schemes for reducing the number of data drive ICs have recently been proposed. FIG. 2 shows one such scheme, a DRD method of implementing the same resolution as the conventional art by halving the number of data drive ICs in such a way as to double the number of gate lines but halve the number of data lines compared to the conventional art.
Referring to FIG. 2, the conventional liquid crystal display driven using the DRD method is configured to drive m (m is a natural number greater than or equal to 2) liquid crystal cells, arranged in one horizontal line, using two gate lines and m/2 data lines. The conventional liquid crystal display is configured to drive the data drive ICs using a 2-dot inversion method in order to minimize flicker and reduce power consumption. Accordingly, two neighboring liquid crystal cells with one data line between them are respectively coupled to two gate lines and charged with data voltages having the same polarity, supplied through the data line. For example, in a specific frame, an R liquid crystal cell and a G liquid crystal cell sharing a first data line D1, among liquid crystal cells arranged in a first horizontal line HL1, may be sequentially charged with positive voltages at the same time as scan pulses are supplied from respective gate lines G1 and G2, an R liquid crystal cell and a B liquid crystal cell sharing a second data line D2, among the liquid crystal cells, may be sequentially charged with negative voltages at the same time as scan pulses are supplied from the respective gate lines G1 and G2, and an R liquid crystal cell and a B liquid crystal cell sharing a third data line D3, among the liquid crystal cells, may be sequentially charged with positive voltages at the same time as scan pulses are supplied from the respective gate lines G1 and G2. An arrow shown in FIG. 2 indicates the charge sequence of the liquid crystal cells coupled to the data lines.
FIG. 3 shows the waveforms of charge voltages in the liquid crystal cells when the liquid crystal cells are charged in the direction of the arrow of FIG. 2. Referring to FIG. 3, the R liquid crystal cells coupled to the first or third gate line G1 or G3 are supplied with a positive voltage (or a negative voltage) which rises (or falls) from a negative voltage (or a positive voltage), and the G liquid crystal cells coupled to the second or fourth gate line G2 or G4 are supplied with a positive voltage (or a negative voltage) which changes from a positive voltage (or a negative voltage). Further, the B liquid crystal cells coupled to the first or third gate line G1 or G3 are supplied with a positive voltage (or a negative voltage) which rises (or falls) from a negative voltage (or a positive voltage), and the B liquid crystal cells coupled to the second or fourth gate line G2 or G4 are supplied with a positive voltage (or a negative voltage) which changes from a positive voltage (or a negative voltage). As known in the art, the amount of charge of liquid crystal cells to which a positive voltage rising from a negative voltage (or a negative voltage falling from a positive voltage) is supplied is smaller than the amount of charge of liquid crystal cells to which a positive voltage changing from a positive voltage (or a negative voltage changing from a negative voltage) is supplied. This is because the rising time of the positive voltage from the negative voltage (or the falling time of the negative voltage from the positive voltage) is long, whereas the rising time of the positive voltage from the positive voltage (or the falling time of the negative voltage from the negative voltage) is short.
Accordingly, in the conventional liquid crystal display using the DRD method, the amount of charge of liquid crystal cells coupled to odd-numbered gate lines (i.e., all the R liquid crystal cells and some of the B liquid crystal cells) is smaller than the amount of charge of liquid crystal cells coupled to even-numbered gate lines (i.e., all the G liquid crystal cells and the remaining B liquid crystal cells). In other words, the R liquid crystal cells are charged relatively weakly, the G liquid crystal cells are charged relatively strongly, and the B liquid crystal cells are alternately charged strongly/weakly on a pixel-by-pixel basis. Here, neither the weakly charged liquid crystal cells nor the strongly charged R and G liquid crystal cells are easily seen, but the alternately charged B liquid crystal cells are easily seen as a vertical line (DIM). Consequently, the conventional liquid crystal display driven using the DRD method is problematic in that picture quality is lowered because of the vertical line (DIM) of a specific color resulting from the difference in charge characteristic.